We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
All-fiber refractive index (RI) sensors are important in chemical and biotechnology applications for a range of superior advantages, such as high sensitivity, low cost, compact size, immunity to electron-magnetic interference and usage in harsh environments. In the past few years, a number of all-fiber RI sensors have been developed, including fiber Bragg grating (FBG) [1–3], long-period grating (LPG) [4–6], D-shaped optical fiber [7–10], microstructure fiber , surface plasmon resonance (SPR) [11, 12], cascaded/hybrid configuration [13, 14] and a variety of fiber interferometers [15–17]. Among them, multimode interferometer (MMI)-based RI optical sensors have attracted great attention recently owing to their distinct advantages of ease of fabrication and low cost. The MMI-based sensors are usually fabricated by splicing special fibers, such as multimode fiber (MMF) , no core fiber (NCF) , photonic crystal fiber (PCF) [20, 21], or using fiber taper technology [22–24]. However, special fibers are expensive and need special splicing program to assemble the individual fibers together. Besides, for some special fiber based MMI structures, part of fiber cladding needs to be removed or thinned [25, 26] to obtain high RI sensitivity, which usually needs additional chemical etching process or fiber tapering technology. And the thinned/tapered fiber inherently has a bad mechanical strength and short service life.
Previously, it has been reported that multimode interference waveguide in bulk glass can be fabricated by a femtosecond laser (fs-laser) . However, the device is not compatible with optical communication systems since it requires complex arrangements of lens to shape and couple the light beams, and the operating wavelength is beyond the communication wavelength. Optical fiber could be a perfect platform to fabricate MMI structures due to its inherent properties of compact size, low cost and naturally compatible with existing optical communication system.
In this paper, we propose and experimentally demonstrate a novel assembly-free fiber inline MMI in a common telecommunication single mode fiber (SMF). The device is based on an in-core negative refractive index modification line (NRIML), fabricated by one-step femtosecond laser direct writing without using any patterned masking or post-processing. Compared with assembly-based MMI and tapered MMI, our proposed NRIML-based MMI has the advantages of simple structure, assembly-free, ease of fabrication, good robustness and low cost. As a RI sensor, a maximum sensitivity of 10675.9 nm/RIU is achieved in the RI range of 1.4484-1.4513.
2. Schematic and fabrication of the MMI
The schematic configuration of the proposed MMI is illustrated in Fig. 1. The MMI structure comprises a section of in-core NRIML, which can be obtained by direct fs-laser writing in the SMF. Usually, to obtain negative index change, relatively high pulse energy for fs-laser writing has to be used . As is known to all, light trends to concentrate on the media with a larger refractive index. Generally, the refractive index of the fiber core is larger than the cladding, which makes the light field tightly confined in the core. However, due to the inducing of the NRIML, the average refractive index of the fiber core decreases. When the refractive index of the fiber core is decreased to be smaller than that of the cladding, the light energy will widely spread into the cladding in the NRIML zone. Thus, multimode interference occurs.
Figure 2 shows the schematic of the fs-laser direct writing system used for NRIML fabrication. The repetition rate, central wavelength, and pulse width of the fs laser (Spectra-Physics, Spirit 1040-8-SHG) are 200 kHz, 520 nm, and 350 fs, respectively. The laser power was adjusted by a plate in combination with a polarizer. A mechanical shutter was employed to control the exposure time. To monitor the stability of the laser power during fabrication, a beam splitter was utilized to split the input laser beam into two with a fixed intensity ratio. One of the split beam was monitored by a power meter, and the other beam passed through a convex and concave lens to produce a high-quality collimated beam. The laser beam was then directed into a microscope objective (Olympus UMPLFL 40X) with a numerical aperture (NA) of 0.65 and focused into the core of a SMF (Yangtze Optical Fiber and Cable Company Ltd.) with a core/cladding diameter of 8.3/125μm. The fiber was mounted on a computer-controlled XYZ translation stage (Newport, Inc.) with a resolution of 0.02 μm. An online monitoring CCD system was used to monitor the sample morphology during the direct writing process. The actual laser energy used for the fabrication was approximately 3μJ/pulse.
3. Principle and spectra characteristics
During the fabrication of the device, the transmission spectrum was real-time monitored by a spectral monitoring system, which included a super-continuum white light source (YSL Super-continuum Source SC-5), and an optical spectrum analyzer (OSA, YOKOGAWA AQ6370C). The fabrication process of the proposed MMI was quite straightforward and involved only one step. When the center of the fiber core was correctly aligned to the focus point of the fs-laser (z-direction), a NRIML can be directly written by scanning the fiber along its core axis (x-direction). The scanning speed used in our experiment was 200 μm/s. Figure 3 shows the microscope image of a small part of the NRIML structure fabricated in the fiber core.
After propagation along the NRIML, the phase difference () of the fundamental mode and any higher-order mode involved in the interference can be written asEq. (2), the wavelength spacing () can be calculated asEq. (3), decreases with the length of NRIML increasing.
The measured transmission of the MMI with different NRIML length of 2.5 mm, 5 mm and 10 mm are shown in Fig. 4(a), from which we can see that the longer NRIML length corresponds to the larger insertion loss. Because when the NRIML becomes longer, the light suffers more scattering and diffraction. It is also evident that with the increase of the NRIML length, the wavelength spacing between two adjacent interference fringes dips decreases, which is good accord with Eq. (3).
In order to analyze the number and the power distribution of the interference modes, the experimental spectra in Fig. 4(a) are taken fast Fourier transform (FFT) to obtain the spatial frequency spectra, as shown in Fig. 5. It is seen in Fig. 5 that there are two or three dominant peaks in the spatial spectra, which means two or three pairs of dominant modes are mainly associated with the interference spectra. The dominant maximum frequency are located at ~0.008, 0.016 and 0.032 nm−1 for MMI length of 2.5, 5 and 10 mm, respectively, which are well in accord with Fig. 4(a) and Eq. (3).
4. Refractive index sensing, simulation and discussion
To characterize the RI response of the structure, one sample with 5 mm NRIML length was immersed in a series of sucrose solutions with different concentrations and the corresponding transmission spectra were recorded by the OSA. When the surrounding RI was increased from 1.3333 to 1.4513, the transmission spectrum evolution of the sample was shown in Fig. 6. It can be clearly seen in Fig. 6 that, with the increase of the RI, the dip wavelengths in the transmission spectra had a significant monotonic red shift, and the wavelength change rate became faster with the RI increasing. The wavelength shifts of the dips A, B and C were chosen to be traced when the MMI was immersed into different solutions.
Figure 5 and 6 show that there are multiple pairs of modes contributing to the complex interference spectrum and the spectral overlapping will likely occur between these mode interferences, so the method of directly tracking individual dips might not be very precise. Discrete Fourier transform (DFT) method  has been demonstrated as a good technique to solve such a problem. For the measured 5-mm-long sensor, the dominant frequencies of the two pairs of interference modes are located at 0.008 and 0.016 nm−1, respectively. Two band-pass DFT filters with frequency range in 0.006-0.01 and 0.014-0.018nm−1, respectively, were chosen to extract each individual mode pair at different surrounding RI. The DFT spectrum evolutions are shown in Fig. 7(a) and 7(b), where sinusoidal components are clearly seen. Dips A’, B’ and C’ in Fig. 7 corresponding to the Dips A, B and C in Fig. 6, respectively. Obviously, the Dips A’ and C’ stem from the same low order mode pairs and the Dip B’ stems from a higher order mode pair.
For comparison, the wavelength shifts of measured MMI dips obtained by direct spectral tracking and extracted by the DFT method are shown in Fig. 8. The wavelength shifts of the selected Dips A, B and C at different surrounding RI using direct spectral tracking are shown in Figs. 8(a)-8(c), where the maximum sensitivities of 10675.9, 5301.7 and 6913.8 nm/RIU are achieved in the RI range of 1.4484-1.4513, 1.4424-1.4484 and 1.4484-1.4513, respectively. Figures 8(d)-8(f) show the wavelength shifts of the selected Dips A’, B’ and C’ obtained with the DFT method, where the maximum sensitivities of 6240.7, 4151.7 and 6655.2 nm/RIU are achieved in the RI range of 1.4484-1.4513, 1.4424-1.4484 and 1.4484-1.4513, respectively. Dips A’ and C’ stem from the same lower interference modes pair (frequency~0.008 nm−1), while Dip B’ stems from a higher modes pair (frequency~0.016 nm−1). Compared to Dip B’, Dips A’ and C’ can work in a higher RI region (1.4484-1.4513) as the higher order interference mode (Dip B’) will disappear first with the RI increasing. Within the same RI range, the sensitivity of Dip B’ is larger than Dips A’ and C’(e.g., in the RI range of 1.4296-1.4484), which is reasonable since the Dip B’ belongs to a higher order mode interference. Without the DFT processing, one cannot find this fact since Figs. 8(a)-8(c) do not shown this.
The beam propagation method (BPM) is utilized to analyze the property of the proposed MMI. In the simulation, the length and the width of NRIML are 5 mm and 5.6 μm, respectively. The refractive indices of the fiber core and the cladding are assumed to be 1.462 and 1.455, respectively, while the RI modification in the NRIML zone is set to be −0.013. Figure 9(a) and (b) give the simulated normalized field distribution of the light propagating in the MMI structure, where one can clearly see that a large number of cladding modes are excited in the NRIML zone and strong coupling occurs. Figure 9(a) shows the simulation with an input wavelength at about 1469 nm (an interference wavelength peak), where the light has a relatively low loss after the NRIML interference. Figure 9(b) shows the simulation with an input wavelength at about 1435nm (an interference wavelength dip), from which we can see that there is almost no light field in the fiber core after the NRIML interference, but widely spreads into the cladding area. The simulated transmission spectra with NRIML length of 2.5 mm, 5 mm and 10 mm are shown in Fig. 4(b). It is obvious that the simulated wavelength spacing between two adjacent interference fringes dips are consistent with the experimental results mentioned above in Fig. 4(a). Figure 9(c) shows the simulated spectral evolution when the surrounding RI varying from 1.33 to 1.45. The simulated and experimental wavelength shifts of the dips at different surrounding RIs are plotted in Fig. 9(d)-9(f), which appear in a good agreement with each other. The small discrepancies between the calculated and experimental results could be due to: (1) the difference between the parameters used in the simulation model and the actual structure (2) the refractive index dispersion, i.e., the refractive index of the sucrose solutions was calibrated using Abbe refractometer (at 589.3 nm), while the sensing experiment carried out in the range of 1400-1600 nm.
In conclusion, we have proposed and experimentally demonstrated a novel assembly-free MMI in SMF for RI measurement. The device is simply fabricated by direct fs-laser writing and the whole fabrication process involves only one step. A maximum sensitivity of 10675.9 nm/RIU were achieved in the RI range of 1.4484-1.4513 experimentally. The BPM method is utilized to analyze the property of the proposed MMI, and the simulated sensitivities and their variation trend exhibit high consistency with the experimental results. The proposed MMI sensor possesses the advantages of assembly-free, robustness, compact size, and high RI sensitivity, which make it promising in chemical and biological applications.
National Natural Science Foundation of China (NSFC) (61775074); National 1000 Young Talents Program, China; 111 Project (No. B07038)
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